Influence of Melt Undercooling on the Microstructure of Fe-Ni Droplets Produced in the Cambridge 6.5 m Drop-Tube

Dilute Fe-Ni alloys have been processed by containerless solidification in a drop-tube, and the resulting microstructures have been characterized mainly by optical metallography. For these alloys a competition exists between primary solidification to the equilibrium bcc alpha(Fe) phase and of the metastable fcc gamma(Fe(Ni)) phase. For all droplets with compositions not more than 5 at. pct Ni, the primary solidification phase is fcc gamma(Fe(Ni)). With less than 5 at. pct Ni the gamma(Fe(Ni)) transforms on solid state cooling to the low temperature bcc alpha(Fe) form giving a Widmanstaetten morphology. With 5 at. pct Ni, however, the final microstructure depends on the droplet size. For large droplets (greater than 600 microns) the microstructure transforms to bcc alpha(Fe). In the small droplets (less than 200 microns) the transformation is suppressed, resulting in a retained gamma(Fe(Ni)) microstructure. For 10 at. pct Ni, the microstructure consists of the metastable alpha(Fe) phase which is in agreement with the literature. For additions of 30 at. pct Ni it is not possible to reach the metastable bcc liquidus so the equilibrium gamma(Fe(Ni)) phase forms from the melt. (Author)